IJC H2 CHEM P2 Teachers copy

INNOVA JUNIOR COLLEGE JC 2 PRELIMINARY EXAMINATION in preparation for General Certificate of Education Advanced Level Higher 2 CANDIDATE NAME CLASS INDEX NUMBER CHEMISTRY Paper 2 Structured Questions Candidates answer on the Question Paper Additional Materials: Data Booklet 9647/02 18 August 2016 2 hours READ THESE INSTRUCTIONS FIRST Write your index number, name and civics group. Write in dark blue or black pen. You may use pencil for any diagrams, graphs or rough working. Do not use staples, paper clips, highlighters, glue or correction fluid. Answer all questions in the space provided. A Data Booklet is provided. You are advised to show all working in calculations. You are reminded of the need for good English and clear presentation in your answers. You are reminded of the need for good handwriting. Your final answers should be in 3 significant figures. You may use a calculator. The number of marks is given in brackets [ ] at the end of each question or part question. At the end of the examination, fasten all your work securely together. This document consists of 17 printed pages and 1 blank page. Innova Junior College [Turn over For Examiner’s Use Section A 1 12 2 17 3 10 4 18 5 15 Significant figures Handwriting Total 72

2  IJC 2016 Prelim/9647/02 [Turn over For Examiner’s Use Answer ALL questions on the spaces provided. 1 Planning Using the information below, you are to write a plan for determining number of molecules of water of crystalisation, n, in barium chloride crystals, BaC l2.nH2O where n = 1,2 or 3. The basis of this investigation is a technique known as precipitation titration with the use of silver nitrate solution, AgNO3(aq). AgCl, Ag2CrO4 and BaCrO4 are sparing soluble salts and relevant information about these salts are given in the table below. compound colour Numerical value of Ksp at 25 O C AgCl white 2.0 × 10−10 Ag2CrO4 red 2.0 × 10−12 BaCrO4 yellow 1.0 × 10−10 Some barium chloride crystals, BaCl2.nH2O, is dissolved to make up 250.0 cm3 of standard solution. Approximately 10 cm 3 of 0.2 mol dm −3 of reagent X solution is added to a portion of this standard solution to precipitate the barium ions before a few drops of K 2CrO4 indicator solution is added. A titration is then carried out on this portion of solution against silver nitrate solution, AgNO 3(aq). Ag 2CrO4 would just precipitate only when almost all of the Cl− ions have been precipitated as AgC l. The titration is repeated until a more reliable average value, V cm3, can be determined. (a) (i) Identify reagent X. [1] Na2SO4(aq) or K2SO4(aq) [1] (ii) A student suggested that BaCrO4 is more soluble than Ag2CrO4 as BaCrO4 has larger numerical value of Ksp than Ag2CrO4. Suggest whether this student’s claim is valid. [1] This claim is not true as both Ag2CrO4 and BaCrO4 are of different unit formula or have different number of ions per formula unit. [1] Alternatively, students can prove from calculations of solubility of the two salts. Solubility of BaCrO4 = = 1.00 × 10−5 mol dm−3 Solubility of Ag2CrO4 = = 7.94 × 10−5 mol dm−3 From the calculations above, Ag2CrO4 has a higher solubility than BaCrO4 despite Ag 2CrO4 having a smaller numerical value of Ksp than BaCrO4. [1] (iii) Suggest why reagent X is added before a few drops of K2CrO4 indicator solution is added. [1] This is to prevent the precipitation of BaCrO 4 that uses up the CrO 42- ions.

3  IJC 2016 Prelim/9647/02 [Turn over For Examiner’s Use (b) You may assume that you are provided with  0.10 mol dm−3 silver nitrate  0.2 mol dm−3 of reagent X solution  10 g of barium chloride crystals, BaCl2.nH2O  Potassium chromate solution, K2CrO4(aq)  The equipment and materials normally found in a school or college laboratory. Your plan should include the following  brief, but specific details of the apparatus you would use, bearing in mind the levels of precision they offer  details, including quantities, for preparation of 250.0 cm 3 of BaCl2 solution from barium chloride crystals, BaCl2.nH2O  essential details of the titration procedure  an outline of how the results obtained, including V cm3, would be used to determine n, in barium chloride crystals (BaCl2.nH2O) Assuming titre volume of AgNO 3(aq) used is 25.00 cm 3 and the volume of BaCl2 solution pipetted is 25.0 cm3, Ag+(aq) + Cl−(aq)  AgCl(s) BaCl2 (aq)  Ba2+(aq) + 2Cl−(aq) Amt of Ag+(aq) = = 0.00250 mol = Amt of Cl−(aq) in 25 cm3 Amt of Cl−(aq) in 250 cm3 = 0.00250 × = 0.0250 mol Amt of BaCl2(aq) in 250 cm3 = 0.0250 mol ÷ 2 = 0.0125 mol Conc of BaCl2(aq) = 0.0125 mol ÷ = 0.0500 mol dm−3 Assuming n = 1, Min mass of BaCl2.nH2O in 250 cm3 = 0.0125 × 226.0 = 2.825 g OR Assuming n = 3, Max mass of BaCl2.nH2O in 250 cm3 = 0.0125 × 262.0 = 3.725 g Thus the suitable mass of BaCl2.nH2O to be weighed ranges 2.825 g to 3.725 g. Preparation of standard solution of BaCl2 1. Weigh 2.825g – 3.275g g of BaCl2.nH2O crystals in a clean and dry weighing bottle using a weighing balance. 2. Transfer the BaCl2.nH2O crystals to a 100 cm3 beaker. 3. Weigh the emptied weighing bottle and the residual BaCl2.nH2O crystals to determine the mass of BaCl2.nH2O crystals dissolved, m grams, by finding the

4  IJC 2016 Prelim/9647/02 [Turn over For Examiner’s Use difference the weighings in (i) and (iii). 4. Add 100 cm3 of de-ionised water to the BaCl2.nH2O crystals in the 100 cm3 beaker. 5. Use a glass stirrer to stir the solution in the 100 cm3 beaker to ensure a homogeneous solution is obtained. 6. Transfer the homogeneous solution carefully to a 250 cm3volumetric flask. 7. Using 50 cm3 of de-ionised water, rinse the beaker, the weighing bottle and the stirrer and transfer the washings carefully to the solution in the 250 cm3volumetric flask. 8. Top up to the mark with de-ionised water, stopper the bottle and shake this solution to obtain a homogeneous solution. Label this solution obtained as FA 2. Titration Procedures 1. Fill up the burette with AgNO3(aq), noting the initial volume reading. 2. Pipette 25.0 cm3 of FA 2 to a 250 cm3 conical flask. 3. Using a 10cm3 measuring cylinder, add 10 cm3 of 0.2 mol dm−3 of Na2SO4 solution to the solution in the conical flask. Shake the mixture and let it equilibrate for about 15 minutes. 4. Add 5 drops of K2CrO4(aq) indicator solution to the mixture in the conical flask. 5. Carry out the titration until permanent pink colour is obtained. (Colour change is from white ppt to pink due of 1 drop of AgNO3(aq) in excess to cause precipitation of red Ag2CrO4) 6. Repeat titration until at least 2 consistent titre readings are obtained (titre volume differ not more than 0.10 cm3). Use the average of these 2 consistent titres to obtain volume, V cm3 for subsequent calculations. Calculation Procedure Amt of Cl− ions in titration (25 cm3) = = 0.000100V mol Amt of Cl− ions in 250 cm3 = 0.000100V x 10 = 0.00100V mol Amt of BaCl2 = 0.00100V ÷ 2 = 0.000500V mol Mass of BaCl2 = 0.000500V × 208.0 = 0.104 V g Mass of water of crystalisation = (m − 0.104 V ) g Amt of water of crystalisation = ((m − 0.104 V ) ÷ 18) mol